This invention relates to improvements in optical circuits and in particular to an N-bit optical comparator for comparing two N-bit binary words. The comparator is especially suitable for use as a building block in a larger optical circuit, such as an all optical telecommunications switch.
N-bit comparators compare two N-bit binary words A and B, where N is greater than 1, and provide as an output at least one of the Boolean functions: A<B, A>B and at least one of NOT (A=B) and A=B. A fully functioned comparator will provide all of these signals as its three outputs. Each signal may take the form of a one bit Boolean signal indicating whether the logical/Boolean function represented by the signal is true or false (e.g. A<B is true or A>B is false). The values true and false can be coded 1 and 0 respectively.
Traditionally comparators for use in high performance computing systems and communications networks have been constructed using discrete digital electronic circuits, with the two words A and B comprising electronic signals and the outputs also comprising electronic signals. A four bit Boolean word can be expressed as a sequence of four binary digits. The circuit can be constructed using a wide variety of small electronic logic circuits, so called “logic gates” which embody Boolean functions such as AND, OR, and NOT. More complex Boolean expressions such as XOR and NAND can be constructed by combining the simpler gates into larger circuits. As the field of digital logic is well established these gates are available at very low costs making the circuits highly economical to produce.
A limitation with digital electronics arises when they are required to process signals at very high speeds. Whilst the logic gates can change state quickly they cannot meet the high speed demands required from modern telecommunications equipment. Whereas telecommunications once only transmitted low bandwidth voice data, it is now required to transmit video information which requires much greater bandwidth. Electronic devices are reaching fundamental limits at these high bandwidths in terms of their power consumption, wiring density and throughput.
Recent trends have seen computing systems and communications with ultra high bandwidths, approaching 160 GB per second or more, being implemented by transmitting the information as optical signals, typically in the form of packets of information, across the network. To take full advantage of the bandwidth, a need has arisen to extract and process information embedded in the signals, such as packet routing information, wholly in the optical domain. This has led the applicant to appreciate that there is a need to develop new and innovative optical logic circuits for use in these systems, including optical comparators.
One known solution to the problem of providing an all optical N-bit comparator for use at ultra-high bandwidths is known from the paper presented by J. M. Martinez et al in the journal IEEE Photonics Technology Letters, Vol. 18 (2006), No. 1, pp. 151-153. This teaches a cascade of SOA-MZI structures. With this approach there is a requirement for N SOA-MZI structures for comparing two N-bit numbers. Optical amplifiers are currently not as low cost to produce as their electronic counterparts, so this need for N structures can result in a relatively high cost circuit when the numbers have many bits (N greater than 6 or so).
An alternative prior art solution is taught in a paper presented by T. Yasui et al in the journal IEEE Journal of Lightwave Technology, Vol 24 (2006), No 2, pp. 723-733. This paper teaches a solution which exploits differential spin excitation in semiconductor multiple quantum wells.
An object of the invention is to provide a comparator which uses all optical components and which can have an architecture that can readily be scaled to cope with N bit numbers having a relatively high number of bits (N greater than 4 or so) without a significant associated cost or complexity increase.